PTC heater assembly bonding

ABSTRACT

An assembly comprising a PTC heater, a body to be heated thereby, and a bonding layer interposed therebetween. The heater is formed from a ceramic type PTC electrical resistance material which will generate heat in response to the flow of electric current therethrough. It has a surface portion through which heat is to be transferred and through which electrical current will flow. The body to be heated has a thermally and electrically conductive surface portion of a shape which substantially matches that of the heater surface portion. The body has a thermal coefficient of expansion differing from that of the heater resistance material. The layer of bonding material is electrically and thermally conductive and is interposed between these surface portions for the transfer of heat and conduction of electric current therethrough. It comprises a mixture of small electrically and thermally conductive particles dispersed throughout a flexible, elastic and creep-resistant high temperature resin material stable at temperatures on the order of at least 150*C. and free of components which tend to degrade the PTC material.

United States Patent [191 Fuller et al.

[ Aug. 5, 1975 1 PTC HEATER ASSEMBLY BONDING [73] Assignees TexasInstruments Incorporated,

Dallas, Tex.

221 Filed: Dec. 26, 1973 21 Appl, No: 428,180

[52] US. Cl. 219/201; 123/119 F; 156/329;

156/331; 219/202; 252/511; 252/514; 338/328 [51] Int. Cl. H051) 1/00[58] Field of Search 219/200-202,

219/207, 209-210, 336, 338, 385, 426, 458, 463, 465; 338/223224, 228;156/325, 329, 331; 261/39 E, 39 A, 23 A; 123/119 F; 252/511, 512, 514

lrish 261/39 E Primary E.\aminer-C. L. Albritton Attorney, Agent, orFirm-James P. McAndrews; John A. l-laug; Russell E, Baumann [57]ABSTRACT An assembly comprising a PTC heater, a body to be heatedthereby, and a bonding layer interposed therebetween. The heater isformed from a ceramic type PTC electrical resistance material which willgenerate heat in response to the flow of electric current therethrough.It has a surface portion through which heat is to be transferred andthrough which electrical current will flow. The body to be heated has athermally and electrically conductive surface portion of a shape whichsubstantially matches that of the heater surface portion. The body has athermal coefficient of expansion differing from that of the heaterresistance material. The layer of bonding material is electrically andthermally conductive and is interposed between these surface portionsfor the transfer of heat and conduction of electric currenttherethrough. It comprises a mixture of small electrically and thermallyconductive particles dispersed throughout a flexible, elastic andcreep-resistant high temperature resin material stable at temperatureson the order of at least 150C. and free of components which tend todegrade the PTC material.

2 Claims, 3 Drawing Figures PATENTEDAUG 5% 3,898,422

illIl-IIII lIIIIIl/II PTC HEATER ASSEMBLY BONDING BACKGROUND OF THEINVENTION This invention relates to PTC heater assembly bonding and moreparticularly to such assemblies in which ceramic-type PTC heaters arebonded to bodies to be heated to elevated temperatures.

Positive temperature coefficient (PTC) resistance materials are widelyutilized as heaters in thermal relays, time-delay relays and otherthermally responsive devices and apparatus. Ceramic-type PTC material iscommonly used for fabrication of such heaters. Typical of these arebarium titanate and related divalent titanates and zirconates whichundergo solid phase changes at particular temperatures. Associated withthese phase changes are abrupt and large changes in the resistivity ofthe materials. These ceramic PTC materials generally include dopantssuch as rare earth metal, antimony or bismuth, or other elements toprovide desired characteristics. For example, lead and strontium may beemployed to increase or decrease the anomaly temperature range of thesematerials.

Such heaters are fixed to a body to be heated so as to be in closeheat-exchange relationship therewith and usually also to provide anelectrically conductive path because the electrical circuit supplyingpower to the heater desirably includes or utilizes the usuallyelectrically conductive body in the circuit. In order to provide thegood thermal and electrical conductivity across the interface betweenthe PTC heater and the body to which it is held fixed in the overallassembly, epoxybased resin materials carrying conductive particlesinterspersed therethrough have generally been used. This interposedlayer of epoxy-conductive particle material forms a good thermal andelectrical bond between the PTC heater face and the face of the body tobe heated. However, the bond is a relatively rigid bond and has certaindisadvantages. There is a substantial difference in the thermalexpansions of this epoxy-based bonding material and the PTC. During heatcycling this causes disruptions of the conducting mechanism in theepoxyconductive material bonding layer. This behavior is especiallyimportant when the entire system is rigidly bonded, i.e, where theforces are well coupled so as to cause displacement of the conductiveparticles in the epoxy from their equilibrium positions in the highlyconducting mode. This momentary particle displacement can cause arc gapsand electrical as well as mechanical degradation of the interfacematerial.

Also, the epoxy and its curing agent tend to degrade the PTC anomaly bycausing a chemical reaction within the bulk of the PTC ceramic. Adecrease in PTC resistance at the operating temperature may causeelectrical run-away and lead to catastrophic breakdown.

Epoxies and related organic synthetic resins belong to a family ofmaterials that have a rated prolonged lifetime at temperatures around150C. At higher temperatures, these organic materials undergokinetically controlled thermal degradation. Since these resin materialsare used as host matrices for the conducting particles, such thermaldegradation will cause destructive effects on the electrical and thermalproperties of the interface per se.

SUMMARY OF THE INVENTION Among the several objects of this invention maybe noted the provision of PTC heater assemblies in which the bondinglayer is not subject to significant electrical and mechanicaldegradation even at elevated operating temperatures in the order ofl50l80C. and even higher; the provision of PTC heater assemblies inwhich no significant degradation of the PTC anomaly takes place evenafter prolonged operation at such elevated temperatures; and theprovision of such PTC heater assemblies which are convenient andeconomical to fabricate and reliable in use. Other objects and featureswill be in part apparent and in part pointed out hereinafter.

Briefly, an assembly of the present invention comprises a heater formedfrom a ceramic-type PTC electrical resistance maerial which willgenerate heat in response to the flow of electric current therethrough.The heater has a surface portion through which heat is to be transferredand through which electrical current will flow. The assembly includes abody which is to be heated and which has a thermally and electricallyconductive surface portion of a shape which substantially matches thatof the heater surface portion and to which heat is to be transferredfrom the heater. The body has a thermal coefficient of expansiondiffering from that of the heater resistance material. The assembly alsohas a layer of electrically and thermally conductive bonding materialinterposed between the surface portions for the transfer of heat andconduction of electric current therethrough. This layer comprises amixture of small electrically and thermally conductive particlesdispersed throughout a flexible, elastic and creepresistant hightemperature resin material stable at temperatures on the order of atleast C. and free of components which tend to degrade the PTC material.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation of a heaterassembly of the present invention on an enlarged scale; and

FIGS. 2 and 3 are similar elevations of alternate embodiments of thepresent invention.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand more particularly to FIG. 1, a thermal relay is generally indicatedat 1. This relay has a conductive metal, e.g., copper or aluminum,housing or body 3 having a thermally and electrically conductivesubstantially flat surface portion 5 to which heat is to be transferredand which serves as an electrode or terminal to carry electrical currentto a generally flat cylindrical or pill-shaped PTC heater 7. A layer ofelectrically and thermally conductive bonding material 9 is interposedbetween body surface portion 5 and inner surface 11 of heater 7. Thebottom is substantially flat or otherwise substantially matches theshape of body portion 5. A resilient electrically conductive spring 13(secured to a conventional case not shown) for relay 1 bears against theouter surface of pill 7 thereby providing an electrical connectionthereto and applying a mechanical force to bias heater 7 against body 3.The top portion of relay 1 is shown in phantom to include at least onerelay terminal 15 and an electrical terminal 17 contacting body 3 forconnection to an electrical circuit to supply power to heater 7.

Layer 9 is relatively thin (e.g., 34 mils thick) and formed of aflexible, elastic and creep-resistant high temperature resin materialstable at temperatures on the order of at least 150C. and free ofcomponents which tend to degrade the ceramic PTC material. This resinhas a major portion by weight of conductive particles, such as silver,silver-copper alloys, graphite, etc., interspersed therethrough. Forexample, about 60-85 percent or more by weight of conductive metalparticles of an average size of about 1 micron is employed in a siliconeor polyimide resin. Typical silicone resins are those obtainable underthe trade designations 525, SR-l55, SR-520, SR-527 and SR-585 fromGeneral Electric. Polyamic acid-solvent mixtures available under thetrade designation Pyre-ML from E. I. Du- Pont de Nemours and Company andwhich heat-cure to polyimide resins are also excellent high temperatureresins for formation of layer 9. Another high temperature resin for thispurpose is that obtainable under the trade designation AI-lO from AmocoChemicals Corp. Other high temperature resin materials, i.e., thoseuseful at temperatures in the order of about 500F. include benzophenone,polyarnide-imide, polybenzimidazoles, polybenzothiazoles,polyethyleneimines, phosphonitrilic and polyester resins.

Bonding layers 9, as above described, form an effective interfacebetween PTC heaters and the thermal relay body, etc., to which it isaffixed. They are sufficiently flexible and elastic to accommodate thedifferential expansion of the heater and body, e.g., in the order of 1percent, as they undergo heat cycling and at elevated temperatures ofl50l80C. or more. They have sufficient adhesion, e.g., a peel strengthof not less than pounds, and are creep resistant, i.e., they do notsignificantly permanently deform when subjected to the conditions ofusage of these assemblies. Further, with the high loading of conductiveparticles they are also highly thermally conductive and have excellentelectrical conductivity, e.g., in the order of 0.1 ohm/- square/mil.They may be applied by silk screening or other conventional applicationprocedures, or preformed as will be discussed hereinafter and applied insheet or other configurations.

Bonding layer 9 thus permits relative movement and differentialexpansion of the heater and the body during heat cycling without loss ofthermal and electrical contact therebetween. The relatively loose orweak mechanical coupling between the substrate or body and the heaterassures that the equilibrium positions of the conducting particles areminimally disturbed and this avoids particle-particle separation whichleads to areing and degradation of the layer. The electrical switchingcharacteristics of layer 9 are as good as those of epoxy resinmaterials. Chemical additives that avoid making the layer rigid and donot effect degradation of the PTC anomaly (amine curing agents areundesirable because of this) may be used, if desired, but are notessential.

Referring now to FIG. 2 a heater assembly bonding, similar to that ofFIG. 1, is illustrated at 1A except the body surface portion 5 isdefined by a projecting ridge and a preformed layer 9A in sheet form isemployed. Such a sheet of silver-particle loaded silicone resin isavailable under the trade designation 850-Consil from Technical WireProducts Inc. The assembly of FIG. 2 allows the PTC pill to expandaxially, limited only by the spring force of 13, while the ridgesomewhat limits movement laterally.

FIG. 3 illustrates still another embodiment 18 of the present invention,quite similar to that of FIG. 2, wherein the body surface portion 5 isrecessed and a layer is a sheet of high temperature resinconductiveparticles formed to fit within the recess, thus improving heat transferbetween the PTC pill and body 3 due to larger surface area in contactwith the PTC ceramic. In this and the other embodiments the spring 13may be utilized to provide sufficient force to maintain the heater inits equilibrium position.

Typical applications of the assemblies of this invention are intime-delay relays where the PTC heater actuates a bimetal plate, and anautomatic choke control where the PTC ceramic heater is used to actuatea bimetal spring that adjusts the fuel/air ratio.

An assembly of this invention utilizing as a bonding layer 9 a siliconeresin heavily loaded (approximately 85 percent) with silver particleswas successfully tested under operating temperatures of about 190C. for1 year.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. An assembly comprising:

a heater formed from a ceramic titanate PTC electrical resistancematerial which will generate heat in response to the flow of electriccurrent therethrough, said heater having a surface portion through whichheat is to be transferred and through which electrical current willflow;

a metal body which is to be heated and which has a thermally andelectrically conductive surface portion substantially matching in shapewith that of the heater surface portion and to which heat is to betransferred from said heater, said body having a thermal coefficient ofexpansion differing from that of the heater resistance material; and

a layer of electrically and thermally conductive material interposedbetween said surface portions for securing said heater and body to eachother and for the transfer of heat and conduction of electric currenttherethrough between said heater and body, said layer comprising amixture of small electrically and thermally conductive particlesselected from the group of silver, silver-copper alloys and graphitematerials dispersed throughout a material selected from the groupconsisting of silicone resin, polyimide resin, benzophenone resin,polyamideimide resins, polybenzimidazole resins, polybenzothiazoleresins, polyethyleneimine resins, phosphonitrilic resins and polyesterresins so that said layer is flexible, elastic and creep-resistant andstable at temperatures on the order of at least C. and free ofcomponents which tend to degrade the PTC heater material.

2. An assembly as set forth in claim 1 wherein said resin material isselected from the group consisting of silicone resin and polyimide resinand wherein said conductive particles comprise from 60 to 85 percent ofthe weight of said bonding layer and have an average particle size ofabout 1 micron.

1. AN ASSEMBLY COMPRISING: A HEATER FORMED FROM A CERAMIC TITANATE PTCELECTRICAL RESISTANCE MATERIAL WHICH WILL GENERATE HEAT IN RESPONSE TOTHE FLOW OF ELECTRIC CURRENT THERETHROUGH, SAID HEATER HAVING A SURFACEPORTION THROUGGH WHICH HEAT IS TO BE TRANSFERRED AND THROUGH WHICHELECTRICAL CURRENT WILL FLOW, A METAL BODY WHICH IS TO BE HEATED ANDWHICH HAS A THERMALLY AND ELECTRICALLY CONDUCTIVE SURFACE PORTIONSUBSTANTIALLY MATCHING IN SHAPE WITH THAT OF THE HEATER SURFACE PORTIONAND TO WHICH HEAT IS TO BE TRANSFERRED FROM SAID HEATER, SAID BODYHAVING A THERMAL COEFFICIENT OF EXPANSION DIFFERING FROM THAT OF THEHEATER RESISTANCE MATERIAL, AND A LAYER OF ELECTRICALLY AND THERMALLYCONDUCTIVE MATERIAL INTERPOSED BETWEEN SAID SURFACE PORTIONS FORSECURING SAID HEATER AND BODY TO EACH OTHER AND FOR THE TRANSFER OF HEATAND CONDUCTION OF ELECTRIC CURRENT THERETHROUGH BETWEEN SAID HEATER ANDBODY, SAID LAYER COMPRISING A MIXTURE OF SMALL ELECTRICALLY ANDTHERMALLY CONDUCTIVE PARTICLES SELECTED FROM THE GROUP OF SILVER,SILVER-COPPER ALLOYS AND GRAPHITE MATERIALS DISPERSED THROUGHOUT AMATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICONE RESIN, POLYIMIDERESIN, BENZOPHENONE RESIN, POLYAMIDE-IMEDE RESINS, POLYBENZIMIDAZOLERESINS, POLYBENZOTHIAZOLE RESINS, POLYETHYLENEIMINE RESINS,PHOSPHONITRILIC RESINS AND POLYESTER RESINS SO THAT SAID LAYER ISFLEXIBLE, ELASTIC AND CREEP-RESIS TANT AND STABLE AT TEMPERATURES ON THEORDER OF AT LEAST 150*C. AND FREE OF COMPONENTS WHICH TEND TO DEGRADETHE PTC HEATER MATERIAL.
 2. An assembly as set forth in claim 1 whereinsaid resin material is selected from the group consisting of siliconeresin and polyimide resin and wherein said conductive particles comprisefrom 60 to 85 percent of the weight of said bonding layer and have anaverage particle size of about 1 micron.